Radiation image read-out method and apparatus

ABSTRACT

In a radiation image read-out method and apparatus, final read-out is carried out for detecting image signals for use in reproduction of a visible image from a recording medium carrying a radiation image recorded thereon, and read-out conditions for the final read-out and/or image processing conditions are adjusted based on image signals for condition adjustment detected by reading out the image information recorded on the recording medium. Image signals in a partial region at a predetermined position on the recording medium are extracted from the image signals for condition adjustment, and a representative value of the image signal for the partial region is calculated. A predetermined discrimination value representing a relationship between a characteristic value of the image signals for condition adjustment and the representative value is calculated, and the read-out conditions for the final read-out and/or the image processing conditions are adjusted based on the discrimination value.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of reading out a radiation imagewherein final read-out is carried out for detecting image signals foruse in reproduction of a visible image from a recording medium such as astimulable phosphor sheet carrying a radiation image recorded thereon,and an apparatus for carrying out the method. This inventionparticularly relates to a method of reading out a radiation image sothat the density of an object region concerned in a visible imagereproduced based on the read-out image signals is prevented fromdiffering between images of the object recorded in different imagerecording directions, and an apparatus for carrying out the method.

2. Description of the Prior Art

When certain kinds of phosphors are exposed to a radiation such asX-rays, α-rays, β-rays, γ-rays, cathode rays or ultraviolet rays, theystore a part of the energy of the radiation. Then, when the phosphorwhich has been exposed to the radiation is exposed to stimulating rayssuch as visible light, light is emitted by the phosphor in proportion tothe stored energy of the radiation. A phosphor exhibiting suchproperties is referred to as a stimulable phosphor.

As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined PatentPublication No. 56(1981)-11395, it has been proposed to use a stimulablephosphor in a radiation image recording and reproducing system.Specifically, a sheet provided with a layer of the stimulable phosphor(hereinafter referred to as a stimulable phosphor sheet) is firstexposed to a radiation passing through an object such as the human bodyto have a radiation image of the object stored thereon. Then, finalread-out is carried out by exposing the stimulable phosphor sheetcarrying the radiation image stored thereon to stimulating rays such asa laser beam which cause the stimulable phosphor sheet to emit light inproportion to the stored radiation energy, and photoelectricallydetecting the emitted light to obtain electric image signals for use inreproduction of a visible image. An image processing is carried out onthe final read-out image signals obtained by the final read-out, and theradiation image of the object is reproduced as a visible image by use ofthe processed image signals on a recording medium such as a photographicfilm, a display device such as a cathode ray tube (CRT), or the like.

In the aforesaid radiation image recording and reproducing system, therange of the level of the radiation energy stored on the stimulablephosphor sheet is caused to fluctuate among radiation images by changesin the object, the image recording portion thereof, radiation dose, orthe like.

However, in the aforesaid radiation image recording and reproducingsystem, the recorded image information, particularly the range of thelevel of the radiation energy or the like, of each radiation imagestored on the stimulable phosphor sheet may be ascertained in advance,and the final read-out may be carried out by use of read-out conditionssuch as a read-out gain and a scale factor adjusted to appropriatevalues in accordance with the recorded image information of eachradiation image. In this case, for each radiation image, it becomespossible to obtain a visible image free from adverse effects of thefluctuation in the range of the level of the radiation energy stored onthe stimulable phosphor sheet and suitable for viewing, particularly fordiagnostic purposes, for example, a visible image wherein the necessaryobject image information is always expressed within the correct densityrange suitable for viewing, particularly for diagnostic purposes.

Also, in the aforesaid radiation image recording and reproducing system,the image processing of the final read-out image signals obtained by thefinal read-out is carried out by use of image processing conditions suchas gradation processing conditions adjusted for each radiation imagebased on the image recording portion of the object such as the head, thechest or the abdomen, and/or the image recording method such as plainimage recording or contrasted image recording so that a visible imagesuitable for viewing, particularly for diagnostic purposes, can beobtained. However, for example, in the case where the final read-out iscarried out without using the read-out conditions adjusted toappropriate values in accordance with the recorded image information ofeach radiation image, the image processing conditions should preferablybe adjusted by considering the recorded image information of eachradiation image, which has been ascertained in advance, besides theimage recording portion of the object and/or the image recording method.In this manner, it becomes possible to obtain a visible image suitablefor viewing, particularly for diagnostic purposes, wherein the necessaryobject image information is expressed within the correct density range.

Ascertaining of the image information recorded on the stimulablephosphor sheet prior to the final read-out and the image processing maybe carried out by use of the method as disclosed in Japanese UnexaminedPatent Publication No. 58(1983)-67240. In the disclosed method,preliminary read-out for ascertaining the recorded image information ofa radiation image stored on the stimulable phosphor sheet is carried outin advance by use of stimulating rays having stimulation energy of alevel lower than the level of the stimulation energy of stimulating raysused in the final read-out for detecting the image signals for use inreproduction of a visible image for viewing, particularly for diagnosticpurposes, and thereafter the final read-out is carried out. The read-outconditions for the final read-out and/or the image processing conditionsare adjusted based on the recorded image information ascertained by thepreliminary read-out, i.e. the preliminary read-out image signalsobtained by the preliminary read-out.

As mentioned above, the level of the stimulating rays used in thepreliminary read-out is lower than the level of the stimulating raysused in the final read out. Specifically, the effective energy of thestimulating rays which the stimulable phosphor sheet receives per unitarea in the preliminary read-out should be lower than the effectiveenergy of the stimulating rays used in the final read-out.

In the case where only the image processing conditions are to beadjusted based on the recorded image information, it is only necessarythat the recorded image information be ascertained prior to the imageprocessing. In this case, since the final read-out image signalsrepresenting the recorded image information have already been detected,the image processing conditions can be adjusted based on the finalread-out image signals, and the preliminary read-out as mentioned aboveneed not necessarily be carried out.

However, in the case where the radiation image read-out conditions forthe final read-out and/or the image processing conditions are adjustedas mentioned above, the density of a region concerned in the objectoften differs between reproduced visible images when radiation images ofthe same object are recorded in different image recording directions.This problem will hereinbelow be described in detail. FIG. 2A shows thecase where an image of the chest is recorded from the front of theobject for diagnosis of the thoracic vertebrae, and FIG. 2B shows thecase where an image of the chest is recorded from the side of theobject. In the case of the front image recording as shown in FIG. 2A,since the thoracic vertebrae K which are in the image-recording regionoverlap the mediastinum through which radiation cannot readily pass, theamount of the radiation stored at the thoracic vertebra image portion ona stimulable phosphor sheet 103 is small, and only a small amount oflight is emitted by the thoracic vertebra image portion on thestimulable phosphor sheet 103 as shown by the histogram of the read-outimage signals in FIG. 3A when the sheet 103 is exposed to stimulatingrays. On the other hand, in the case of the side image recording asshown in FIG. 2B, since the thoracic vertebrae K overlap the lungsthrough which radiation can readily pass, the amount of radiation storedat the thoracic vertebra image portion on a stimulable phosphor sheet103 is large, and a large amount of light is emitted by the thoracicvertebra image portion on the stimulable phosphor sheet 103 as shown bythe histogram of the read-out image signals in FIG. 3B when the sheet103 is exposed to stimulating rays. Since the maximum value Smax and theminimum value Smin of the read-out image signals detected from thestimulable phosphor sheet do not much differ between the front imagerecording and the side image recording, the read-out conditions for thefinal read-out and/or the image processing conditions adjusted by amethod of adjustment based on the maximum value Smax and the minimumvalue Smin of the read-out image signals as one of the conventionalmethods of adjustment based on the recorded image information becomeapproximately equal between the front image recording and the side imagerecording. Therefore, when the image read-out is carried out and avisible image is reproduced by use of the read-out conditions and/or theimage processing conditions adjusted in this manner, the density of thethoracic vertebra image portion becomes comparatively low in thereproduced visible image obtained in the case of the front imagerecording, and becomes comparatively high in the case of the side imagerecording.

Besides the aforesaid image recording portion of the object and theaforesaid image recording directions, the problems as mentioned abovegenerally arise between the case where an object portion exhibiting aparticularly high or low radiation absorptivity overlaps animage-recording region of an object in the course of image recording andthe case where such an object portion does not overlap the object regionconcerned, depending on the image recording direction of the object.

If the image density of the object region concerned differs betweenradiation images recorded in different image recording directions of theobject, it is not always possible to accurately make diagnosis by theutilization of the radiation images.

The problems as mentioned above may arise also when a radiation imagerecorded on a recording medium other than the stimulable phosphor sheetis read out to reproduce a visible image.

SUMMARY OF THE INVENTION

The primary object of the present invention is to provide a method ofreading out a radiation image wherein, in the course of reproducing avisible radiation image by reading out a radiation image from arecording medium such as a stimulable phosphor sheet carrying theradiation image of an object recorded thereon in a different imagerecording direction, the density of a region concerned in the reproducedvisible image is maintained constant regardless of the image recordingdirection.

Another object of the present invention is to provide a method ofreading out a radiation image, which is suitable for obtaining areproduced visible image having an improved image quality, particularlya high diagnostic efficiency and accuracy.

The specific object of the present invention is to provide an apparatusfor carrying out the method of reading out a radiation image.

A further object of the present invention is to provide a method ofdiscriminating the image recording direction of the object.

The present invention provides a method of reading out a radiation imagein which final read-out is carried out for detecting image signals foruse in reproduction of a visible image from a recording medium carryinga radiation image recorded thereon, and read-out conditions for thefinal read-out and/or image processing conditions for an imageprocessing of final read-out image signals detected by the finalread-out are adjusted based on image signals for condition adjustmentdetected by reading out the image information recorded on the recordingmedium,

wherein the improvement comprises the steps of:

(i) extracting image signals in a partial region at a predeterminedposition on said recording medium from said image signals for conditionadjustment, and calculating a representative value of the image signalfor said partial region,

(ii) calculating a predetermined discrimination value representing arelationship between a characteristic value of said image signals forcondition adjustment and said representative value, and

(iii) adjusting said read-out conditions for the final read-out and/orsaid image processing conditions based on said discrimination value.

The present invention also provides a radiation image read-out apparatuscomprising a final read-out system provided with a means for emittinglight to a recording medium carrying a radiation image recorded thereon,a means for moving said recording medium, and a light detection meansfor photoelectrically detecting an amount of light emission by saidrecording medium upon exposure to said light and obtaining image signalsfor use in reproduction of a visible image, and a system for adjustingread-out conditions for the final read-out and/or image processingconditions for an image processing of final read-out image signals,which are detected by the final read-out, based on image signals forcondition adjustment detected by reading out the image informationrecorded on the recording medium,

wherein the improvement comprises constituting said system for adjustingsaid read-out conditions for the final read-out and/or said imageprocessing conditions by:

(i) a means for extracting image signals in a partial region at apredetermined position on said recording medium from said image signalsfor condition adjustment, and calculating a representative value of theimage signal for said partial region,

(ii) a means for calculating a predetermined discrimination valuerepresenting a relationship between a characteristic value of said imagesignals for condition adjustment and said representative value, and

(iii) a means for adjusting said read-out conditions for the finalread-out and/or said image processing conditions based on saiddiscrimination value.

With the method of and apparatus for reading out a radiation imagewherein the image signal in the partial region at a predeterminedposition on the recording medium are extracted from the image signalsfor condition adjustment, the representative value of the image signalsin the partial region is calculated, and the read-out conditions for thefinal read-out and/or the image processing conditions are adjusted basedon the predetermined discrimination value representing the relationshipbetween the representative value and a characteristic value of the imagesignals for condition adjustment, it is possible to make constant thedensity of the image region concerned in the reproduced visible imageregardless of the image recording direction of the object.

The present invention further provides a method of discriminating theimage recording direction of the object in a radiation image read-outmethod comprising the steps of extracting image signals in a partialregion at a predetermined position on a recording medium from said imagesignals, and calculating a representative value of the image signal forsaid partial region, and calculating a predetermined discriminationvalue representing a relationship between a characteristic value of saidimage signals and said representative value.

As the aforesaid representative value, it is possible to use the meanvalue, the maximum value, the minimum value or the like of the imagesignals in the partial region. For example, in the case where the meanvalue Save of the image signals in the partial region is used as therepresentative value and the maximum value Smax and the minimum valueSmin are used as the characteristic values of the image signals forcondition adjustment, a value Q expressed as

    Q=(Save-Smin)/(Smax-Smin)

or the like may be used as the discrimination value.

By the term "recording medium" as used herein is meant a medium capableof recording a radiation image thereon, such as a stimulable phosphorsheet. However, the recording medium is not limited to the stimulablephosphor sheet.

By the term "final read-out" as used herein is meant the operation ofdetecting (reading out) the image signals for use in reproduction of avisible image from the recording medium, and this term is used in thissense also for the recording medium other than the stimulable phosphorsheet.

By the term "read-out conditions" as used herein are meant variousconditions affecting the relationship between the input and output of aread-out means, for example, the relationship between the input to aphotoelectric read-out means (i.e. the amount of light emitted by thestimulable phosphor sheet in the read-out step) and the output of teephotoelectric read-out means (i.e. the levels of the image signals) inthe case where the recording medium is the stimulable phosphor sheet.For example, the read-out conditions mean the read-out gain(sensitivity), the scale factor (latitude) or the like.

By the term "image processing conditions" as used herein are meantvarious conditions affecting the relationship between the input andoutput of an image processing means, for example, gradation processingconditions, frequency response processing conditions, or the like.

The term "image signals for condition adjustment" as used herein meansthe image signals which are utilized for adjusting the read-outconditions for the final read-out and/or the image processing conditionsand which are obtained by reading out the image information recorded onthe recording medium. For example, the aforesaid preliminary read-outimage signals or the final read-out image signals may be utilized as theimage signals for condition adjustment. However, the image signals forcondition adjustment are not limited to the preliminary read-out imagesignals or the final read-out image signals. In the case where thepreliminary read-out image signals are utilized as the image signals forcondition adjustment, the read-out conditions for the final read-outand/or the image processing conditions can be adjusted based thereon.However, in the case where the final read-out image signals are used asthe image signals for condition adjustment, only the image processingconditions can be adjusted based thereon.

The term "adjusting based on a discrimination value" as used hereinembraces the case where the read-out conditions for the final read-outand/or the image processing conditions are adjusted on the basis of onlythe discrimination value and the case where the conditions are adjustedon the basis of the discrimination value and other factors such as theimage recording portion of the object, the image recording method, or anappropriate characteristic value of the recorded image information.

Adjustment of the read-out conditions for the final read-out and/or theimage processing conditions may be carried out in any manner insofar asthe conditions are adjusted based on the discrimination value, i.e. bythe utilization of the discrimination value. Thus the method ofadjusting the conditions is not limited to a specific one.

Normally, images including a specific region and recorded in differentimage recording directions have patterns different from each other.Specifically, in the case of the image recorded by front image recordingas shown in FIG. 2A for example, the thoracic vertebrae K overlap themediastinum portion including the heart and the main arteries throughwhich radiation cannot readily pass, and the thoracic vertebra imageportion constitutes a portion of a small light emission amount in thehistogram of the read-out image signals within the irradiation field asshown in FIG. 3A. On the other hand, in the case of the image recordedby side image recording as shown in FIG. 2B, the thoracic vertebrae K donot overlap the heart and the main artery, but instead overlap the lungsthrough which radiation can readily pass, and the thoracic vertebraimage portion exhibits a light emission amount larger than at thediaphragm and the apexes of the lungs (i.e. the lung portions close tothe shoulder) in the histogram as shown in FIG. 3B. Further, thediaphragm portion and the lung apex portions are disposed at positionsdeviated from the partial region P shown. Therefore, in this case, whenthe partial region P for extraction of the image signals for conditionadjustment is disposed approximately at the center of the image, theamounts of light emission by the partial region P become as a wholelarger, i.e. the mean value Save becomes larger, in the image shown inFIG. 2B than in the image shown in FIG. 2A. Accordingly, for example,the aforesaid discrimination value Q becomes larger in the case of theimage shown in FIG. 2B than in the case of the image shown in FIG. 2A,and front image recording and side image recording can be indirectlydiscriminated from each other by calculating the discrimination value Q.The read-out conditions for the final read-out and/or the imageprocessing conditions are adjusted to decrease the density of thereproduced visible image when the discrimination value Q iscomparatively large, and to increase the density of the visible imagewhen the discrimination value Q is comparatively small. Thus the densityof the thoracic vertebra image portion as the region concerned can bemaintained identical between the case of front image recording and thecase of the side image recording.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing an example of the radiation imagerecording and reproducing system wherein an embodiment of the method ofreading out a radiation image in accordance with the present inventionis employed,

FIGS. 2A and 2B are schematic views showing examples of radiation imagesrecorded in different image recording directions of an object,

FIGS. 3A and 3B are graphic examples of histograms of the read-out imagesignals detected from stimulable phosphor sheets on which imagerecording has been carried out in different image recording directionsof the object, and

FIG. 4 is a block diagram showing in detail a part of the radiationimage recording and reproducing system shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will hereinbelow be described in further detailwith reference to the accompanying drawings.

Referring to FIG. 1, a radiation image recording and reproducing systembasically comprises a radiation image recording section 20, apreliminary read-out section 30, a final read-out section 40, and animage reproducing section 50. At the radiation image recording section20, radiation 102 is emitted by a radiation source 100 constituted by anX-ray tube or the like toward an object 101. A stimulable phosphor sheet103 for storing radiation energy thereon is placed at the positionexposed to the radiation 102 passing through the object 101, and aradiation image of the object 101 is stored on the stimulable phosphorsheet 103.

The stimulable phosphor sheet 103 carrying the radiation image of theobject 101 stored thereon is sent to the preliminary read-out section 30by a sheet conveyance means 110 constituted by a conveyor roller or thelike. At the preliminary read-out section 30, a laser beam 202 producedby a laser beam source 201 is first passed through a filter 203 forcutting off light having a wavelength within a range identical with therange of the wavelength of the light emitted by the stimulable phosphorsheet 103 upon stimulation thereof by the laser beam 202. Then, thelaser beam 202 is one-dimensionally deflected by a light deflector 204such as a galvanometer mirror and directed onto the stimulable phosphorsheet 103 by a plane reflection mirror 205. The laser beam source 201 isselected so that the laser beam 202 emanated thereby has a wavelengthdistribution different from and far apart from the wavelengthdistribution of the light emitted by the stimulable phosphor sheet 103upon stimulation thereof. While the laser beam 202 impinges upon thestimulable phosphor sheet 103, the stimulable phosphor sheet 103 ismoved in the direction as indicated by the arrow 206 (i.e. in thesub-scanning direction) by a sheet conveyance means 210 constituted byconveyor rollers or the like, and thus the overall surface of thestimulable phosphor sheet 103 is exposed to and scanned by the laserbeam 202. The power of the laser beam source 201, the beam diameter ofthe laser beam 202, the scanning speed of the laser beam 202, and themoving speed of the stimulable phosphor sheet 103 are selected so thatthe level of the stimulation energy of the laser beam 202 forpreliminary read-out is lower than the level of the stimulation energyof the laser beam for the final read-out carried out at the finalread-out section 40.

When exposed to the laser beam 202 as mentioned above, the stimulablephosphor sheet 103 emits light in proportion to the radiation energystored thereon, and the emitted light enters a light guide member 207which may be of a shape and a material as disclosed in U.S. Pat. No.4,346,295. The light is guided inside of the light guide member 207through total reflection, projected from a light output face of thelight guide member 207 and received by a photodetector 208 constitutedby a photomultiplier or the like. The light receiving face of thephotodetector 208 is closely contacted with a filter for transmittingonly light having the wavelength distribution of the light emitted bythe stimulable phosphor sheet 103 and cutting off the light having thewavelength distribution of the stimulating rays, so that thephotodetector 208 can detect only the light emitted by the stimulablephosphor sheet 103 upon stimulation thereof. The light detected by thephotodetector 208 is converted into electric signals carrying the imageinformation stored on the stimulable phosphor sheet 103, and amplifiedby an amplifier 209. The signals generated by the amplifier 209 aredigitized by an A/D converter 211, and sent as preliminary read-outimage signals Sp to a final read-out control circuit 314 at the finalread-out section 40. On the basis of the preliminary read-out imagesignals Sp, the final read-out control circuit 314 calculates a read-outgain setting value (a), a scale factor setting value (b), and agradation processing condition setting value (c) as one of imageprocessing condition setting values by means of histogram analysis orthe like.

After the preliminary read-out is finished, the stimulable phosphorsheet 103 is sent to the final read-out section 40. At this section, alaser beam 302 produced by a laser beam source 301 is first passedthrough a filter 303 for cutting off light having a wavelength withinthe range identical with the range of the wavelength of the lightemitted by the stimulable phosphor sheet 103 upon stimulation thereof bythe laser beam 302. Then, the beam diameter of the laser beam 302 isstrictly adjusted by a beam expander 304. The laser beam 302 is thendeflected by a light deflector 305 formed of a galvanometer mirror orthe like, and is made to impinge upon the stimulable phosphor sheet 103by a plane reflection mirror 306. Between the light deflector 305 andthe plane reflection mirror 306 is disposed an fθ lens 307 formaintaining the beam diameter of the laser beam 302 uniform in thecourse of the scanning of the laser beam 302 on the stimulable phosphorsheet 103. While the laser beam 302 impinges upon the stimulablephosphor sheet 103, the stimulable phosphor sheet 103 is moved in thedirection as indicated by the arrow 308 (i.e. in the sub-scanningdirection) by a sheet conveyance means 320 constituted by conveyorrollers or the like and, consequently, the overall area of thestimulable phosphor sheet 103 is exposed to and scanned by the laserbeam 302. Upon exposure to the laser beam 302, the stimulable phosphorsheet 103 emits light in proportion to the radiation energy storedthereon, and the light emitted enters a light guide member 309 which ismade of the same material and has the same configuration as the lightguide member 207 used for the preliminary read-out. The light emitted bythe stimulable phosphor sheet 103 is guided inside of the light guidemember 309 through total reflection, projected from the light outputface of the light guide member 309 and received by a photodetector 310constituted by a photomultiplier or the like. The light receiving faceof the photodetector 310 is closely contacted with a filter forselectively transmitting only the light having the wavelengthdistribution of the light emitted by the stimulable phosphor sheet 103,so that the photodetector 310 can detect only the light emitted thereby.

The output of the photodetector 310 photoelectrically detecting thelight emission representing the radiation image stored on the stimulablephosphor sheet 103 is amplified to an appropriate level by an amplifier311 the read-out gain of which has been adjusted by the read-out gainsetting value (a) calculated by the control circuit 314. The amplifiedelectric signals are fed to an A/D converter 312 which converts theelectric signals into digital signals by use of a scale factor which hasbeen adjusted by the scale factor setting value (b) to suit the width ofsignal fluctuation. The digital signals, i.e. the final read-out imagesignals thus obtained are fed to a signal processing circuit 313, inwhich they are subjected to a gradation processing (signal processing)based on the gradation processing condition setting value (c) so as toobtain a visible radiation image suitable for viewing, particularly fordiagnostic purposes, and are output as read-out image signals (finalread-out image signals) So.

The final read-out image signals So generated by the signal processingcircuit 313 are fed to a light modulator 401 at the image reproducingsection 50. At the image reproducing section 50, a laser beam 403produced by a reproducing laser beam source 402 is modulated by thelight modulator 401 on the basis of the final read-out image signals Soreceived from the signal processing circuit 313, and is made to impingeupon a photosensitive material 405 such as a photographic film by ascanning mirror 404 for scanning the photosensitive material 405 by thelaser beam 403. At this time, the photosensitive material 405 is movednormal to the aforesaid scanning direction, i.e. in the direction asindicated by the arrow 406. Accordingly, the radiation image representedby the final read-out image signals So is recorded on the photosensitivematerial 405. For reproducing the radiation image, it is possible to useany other appropriate method such as the aforesaid display on a CRT.

The technique of making constant the density of the image regionconcerned in the reproduced visible image in accordance with the presentinvention even through the image recording direction of the object 101is changed will be described hereinbelow. The preliminary read-out imagesignals Sp generated by the A/D converter 211 are sent to the finalread-out control circuit 314 as mentioned above and to a densitycorrection control circuit 500. FIG. 4 shows in detail the configurationof the density correction control circuit 500. As shown in FIG. 4, asignal extracting section 501 of the density correction control circuit500 extracts only the image signals in a partial region P at thepredetermined position on the stimulable phosphor sheet 103 from the fedpreliminary read-out image signals Sp. The partial region P is specifiedby a region setting section 502. For example, in the case of reading outa radiation image of the thoracic vertebrae as mentioned above, thepartial region P is specified to a size of approximately, 5cm×5cm or7cm×7cm at the center of the stimulable phosphor sheet 103 as shown inFIGS. 2A and 2B. The case of reading out the thoracic vertebra imagewill hereinbelow be described by way of example. Preliminary read-outimage signals Sp' extracted as mentioned above only in the partialregion P are sent to an analysis section 503. The analysis section 503creates a histogram of the thus extracted preliminary read-out imagesignals Sp', and calculates a mean value Save thereof. The mean valueSave as a representative value indicating the amount of light emissionby the partial region P is sent to an operating section 504. Theoperating section 504 also receives the maximum value Smax and theminimum value Smin of the whole preliminary read-out image signals Spwhich are calculated by the final read-out control circuit 314. Theoperating section 504 calculates a discrimination value Q expressed as

    Q=(Save-Smin)/(Smax-Smin)

from the three fed values. The discrimination value Q is fed to acomparing section 505, which compares the discrimination value Q with areference value R received from a reference value setting section 506.When the discrimination value Q is larger than the reference value R,the comparing section 505 feeds a correction signal T to a read-out gaincorrecting circuit 507. Upon receiving the correction signal T, theread-out gain correcting circuit 507 corrects the read-out gain settingvalue (a), which has been calculated as mentioned above by the finalread-out control circuit 314, so as to decrease the read-out gain. Asmentioned above, when the image of the thoracic vertebrae K recorded byfront image recording and the image thereof recorded by side imagerecording are compared with each other, the discrimination value Q iscomparatively larger in the case of side image recording than in thecase of front image recording. Also, when the read-out conditions forthe final read-out and/or the image processing conditions are constant,the density at the image portion of the thoracic vertebrae K in thereproduced visible image becomes higher in the case of side imagerecording than in the case of front image recording. To eliminate thisproblem, the read-out gain is decreased by correcting as mentioned abovein the case of side image recording. As a result, the levels of thefinal read-out image signals So become generally low, and the density ofthe visible radiation image reproduced on the photosensitive material405 becomes low as a whole. Accordingly, the density of the imageportion of the thoracic vertebrae K in the visible image reproduced inthe case of side image recording is adjusted to be equal to the densityof the image portion of the thoracic vertebrae K in the visible imagereproduced in the case of front image recording. The appropriatecorrection amount of the read-out gain can be determined experimentally.

In the aforesaid embodiment, the final read-out for the radiation imagerecorded by front image recording is carried out without changing theread-out gain as adjusted by the final read-out control circuit 314, andthe read-out gain is corrected to a smaller value when reading out theradiation image recorded by side image recording. Conversely, the finalread out for the radiation image recorded by side image recording may becarried out without changing the read-out gain as adjusted by the finalread-out control circuit 314, and the read-out gain may be corrected toa larger value when reading out the radiation image recorded by frontimage recording. Also, instead of correcting the read-out gain asadjusted by the final read-out control circuit 314 as in the aforesaidembodiment, the read-out gain setting value (a) calculated by the finalread-out control circuit 314 may be increased or decreased in accordancewith the aforesaid discrimination value Q. In order to adjust thedensity of the reproduced visible image, instead of changing theread-out gain as mentioned above, the conditions of the scale factor inthe A/D converter 312 may be changed, or the gradation processingconditions in the signal processing circuit 313 may be changed. Or,these methods of adjusting the density may be employed in combinationwith each other.

The gradation processing conditions are usually expressed by anon-linear gradation curve. For example, in the case where the gradationprocessing conditions determined by the final read-out control circuit314 are corrected by the correction signal T, the gradation curve may beshifted vertically or horizontally in accordance with the correctionsignal T, or may be rotated around a predetermined position on thegradation curve, so that the density of the region concerned in thereproduced visible image is made constant.

The density of the thoracic vertebra image portion, i.e theimage-recording region, is made constant in the manner as mentionedabove between the cases of front image recording and side imagerecording of the thoracic vertebra image. However, the method of readingout a radiation image in accordance with the present invention isapplicable in the same manner also to the case where a radiation imageis read out from the stimulable phosphor sheet carrying a radiationimage of the other object portions such as the lumbar vertebrae recordedthereon, or the stimulable phosphor sheets on which image recording hasbeen carried out in various different image recording directions of theobject, for example, by front image recording and oblique imagerecording. Specifically, in the images recorded in different imagerecording directions of the object, the partial region P where theaforesaid discrimination value Q is clearly different is usuallypresent. Therefore, the position and the size of the partial region Pand the reference value R may be adjusted to appropriate values in eachcase. The position and the size of the partial region P and thereference value R may be adjusted manually in accordance with the imagerecording portion and the image recording direction of the image whichis to be read out. Or, in the case where the information on the positionand the size of the partial region P and the reference value R isentered as an image recording menu to the read-out apparatus, theposition and the size of the partial region P and the reference value Rmay be set automatically based on the entered information at the regionsetting section 502 and the reference value setting section 506.

As the discrimination value utilized for indirectly discriminating theimage recording direction of the object, instead of the aforesaid valueexpressed as

    Q=(Save-Smin)/(Smax-Smin),

it is also possible to utilize a value expressed as

    (Smax-Save)/(Smax-Smin),

a value calculated by substituting the maximum value or the minimumvalue of the extracted preliminary read-out image signals Sp' for Savein either one of these formulas, or the like.

In the radiation image recording and reproducing system shown in FIG. 1,the preliminary read-out section and the final read-out section aredisposed independently. However, as disclosed in, for example, JapaneseUnexamined Patent Publication No. 58(1983)-67242, a single read-outsystem may be used for the preliminary read-out and the final read-out.In this case, after the preliminary read-out is finished, the stimulablephosphor sheet is returned to the read-out system by a sheet conveyancemeans and then the final read-out is carried out. In the preliminaryread-out step, the stimulation energy of the stimulating rays isadjusted by a stimulating ray energy adjusting means to be lower thanthe stimulation energy of the stimulating rays used in the finalread-out. The present invention is also applicable to such a case.

The present invention is applicable also to the case of subdivisionimage recording wherein a single stimulable phosphor sheet is dividedinto, for example, two subdivisions and image recording is carried outat each subdivision. In the case of the subdivision image recording, thepresent invention may be applied to each of the subdivisions.

In the aforesaid radiation image recording and reproducing system, theimage signals for condition adjustment are obtained by the preliminaryread-out, and the read-out conditions for the final read-out and/or theimage processing conditions are adjusted based on the discriminationvalue representing the relationship between the characteristic value ofthe image signals for condition adjustment and the aforesaidrepresentative value of the image signals in the partial regionextracted from the image signals for condition adjustment. However, in aradiation image recording and reproducing system wherein the preliminaryread-out is not carried out, the final read-out image signals may beutilized as the image signals for condition adjustment. In this case,the final read-out image signals may be stored in a storage means, andthe image processing conditions may be adjusted based on thediscrimination value calculated in the same manner as mentioned above.Then, the final read-out image signals may be read from the storagemeans, and the image processing of the final read-out image signals maybe carried out by use of the thus adjusted image processing conditions.

As mentioned above, with the method of reading out a radiation image inaccordance with the present invention, even though the image recordingdirection of the object is different, it is possible to make constantthe density of the image region concerned in the reproduced visibleimage, and to markedly improve the image quality of the visibleradiation image, particularly the diagnostic efficiency and accuracy.

It should be understood that the present invention can be modified invarious manners within its scope, and is not limited to the aforesaidembodiments.

I claim:
 1. A method of reading out a radiation image in which finalread-out is carried out for detecting image signals for use inreproduction of a visible image for a recording medium carrying aradiation image of an object recorded thereon, and read-out conditionsfor the final read-out and/or image processing conditions for an imageprocessing of final read-out image signals detected by the finalread-out are adjusted based on image signals for condition adjustmentdetected by reading out the image information recorded on the recordingmedium,wherein the improvement comprises the steps of: (i) extractingimage signals in a partial region at a predetermined position on saidrecording medium from said image signals for condition adjustment, andcalculating a representative value of the image signal for said partialregion, (ii) calculating a predetermined discrimination valuerepresenting a relationship between a characteristic value of said imagesignals for condition adjustment and said representative value, andrepresentative an image recording direction of the object, and (iii)adjusting said read-out conditions for the final read-out and/or saidimage processing conditions based on said discrimination value.
 2. Amethod as defined in claim 1 wherein said representative value is a meanvalue Save of the image signals for condition adjustment in said partialregion, the maximum value Smax and the minimum value Smin of said imagesignals for condition adjustment are used as the characteristic valuesof said image signals for condition adjustment, and said discriminationvalue is expressed as

    (Save-Smin)/(Smax-Smin).


3. A method as defined in claim 1 wherein said representative value is amean value Save of the image signals for condition adjustment in saidpartial region, the maximum value Smax and the minimum value Smin ofsaid image signals for condition adjustment are used as thecharacteristic values of said image signals for condition adjustment,and said discrimination value is expressed as

    (Smax-Save)/(Smax-Smin).


4. A method as defined in claim 1 wherein preliminary read-out iscarried out for ascertaining the image information recorded on saidrecording medium prior to said final read-out, and image signalsobtained by said preliminary read-out are utilized as said image signalsfor condition adjustment.
 5. A method as defined in claim 1 wherein saidrecording medium is composed of a stimulable phosphor.
 6. A radiationimage read-out apparatus comprising a final read-out system providedwith a means for emitting light to a recording medium carrying aradiation image of an object recorded thereon, a means for moving saidrecording medium, and a light detection means for photoelectricallydetecting an amount of light emission by said recording medium uponexposure to said light and obtaining image signals for use inreproduction of a visible image, and a system for adjusting read-outconditions for the final read-out and/or image processing conditions foran image processing of final read-out image signals, which are detectedby the final read-out, based on image signals for conditions adjustmentdetected by reading out the image information recorded on the recordingmedium,wherein the improvement comprises constituting said system foradjusting said read-out conditions for the final read-out and/or saidimage processing conditions by: (i) a means for extracting image signalsin a partial region at a predetermined position on said recording mediumfrom said image signals for condition adjustment, and calculating arepresentative value of the image signal for said partial region, (ii) ameans for calculating a predetermined discrimination value representinga relationship between a characteristic value of said image signals forcondition adjustment and said representative value, and representing animage recording direction of the object, and (iii) a means for adjustingsaid read-out conditions for the final read-out and/or said imageprocessing conditions based on said discrimination value.
 7. Anapparatus as defined in claim 6 wherein said representative value is amean value Save of the image signals for condition adjustment in saidpartial region, the maximum value Smax and the minimum value Smin ofsaid image signals for condition adjustment are used as thecharacteristic values of said image signals for condition adjustment,and said discrimination value is expressed as

    (Save-Smin)/(Smax-Smin).


8. An apparatus as defined in claim 6 wherein said representative valueis a mean value Save of the image signals for condition adjustment insaid partial region, the maximum value Smax and the minimum value Sminof said image signals for condition adjustment are used as thecharacteristic values of said image signals for condition adjustment,and said discrimination value is expressed as

    (Smax-Save)/(Smax-Smin).


9. An apparatus as defined in claim 6 wherein the apparatus furthercomprises a preliminary read-out system provided with a means foremitting light to said recording medium, a means for moving saidrecording medium, and a light detection means for photoelectricallydetecting the amount of light emission by said recording medium uponexposure to said light and obtaining image signals, and said system foradjusting said read-out conditions for the final read out and/or saidimage processing conditions utilizes said image signals obtained by saidpreliminary read-out as said image signals for condition adjustment. 10.An apparatus as defined in claim 6 wherein said recording medium iscomposed of a stimulable phosphor, and said means for emitting light tosaid recording medium carrying a radiation image recorded thereon is ameans for emitting stimulating rays which cause said stimulable phosphorto emit light in proportion to radiation energy stored on saidstimulable phosphor.
 11. A method of discriminating the image recordingdirection of an object in a radiation image read-out method whereinread-out is carried out for detecting image signals for use inreproduction of a visible image from a recording medium carrying aradiation image of the object recorded thereon comprising the steps ofextracting image signals in a partial region at a predetermined positionon a recording medium from said image signals, and calculating arepresentative value of the image signal for said partial region, andcalculating a predetermined discrimination value representing arelationship between a characteristic value of said image signals andsaid representative value.
 12. A method as defined in claim 11 whereinsaid representative value is a mean value Save of the image signals insaid partial region, the maximum value Smax and the minimum value Sminof said image signals are used as the characteristic values of saidimage signals, and said discrimination value is expressed as

    (Save-Smin)/(Smax-Smin).


13. A method as defined in claim 11 wherein said representative value isa mean value Save of the image signals in said partial region, themaximum value Smax and the minimum value Smin of said image signals areused as the characteristic values of said image signals, and saiddiscrimination value is expressed as

    (Smax-Save)/(Smax-Smin).